The present invention relates generally to electronic ballasts used for operating gas discharge lamps. More particularly, the present invention pertains to methods and systems for eliminating striations in gas discharge lamps.
A fluorescent lamp is a type of gas discharge lamp having a fluorescent phosphor coating the inside surface of the lamp's sealed tube. The tube may contain a small amount of mercury and an inert gas such as argon. Both ends of the tube have electrodes, often made of tungsten. Initially, as power is delivered to the lamp, the electrodes become thermally agitated and emit electrons. These electrons impact and ionize the inert gas in the tube which results in the formation of a plasma (a phase of matter different from solid, liquid, or gas that readily facilitates current conduction because of its low impedance).
The current now flowing between the electrodes, as a result of the potential difference between the electrodes and the plasma, cause the mercury to ionize and release ultraviolet radiation. The ultraviolet radiation is absorbed by the phosphor coating and, then, the phosphor coating radiates in the visible light spectrum, i.e. produces visible light waves. Through this process the fluorescent lamp efficiently converts electrical energy into visible light.
It is important to recognize that the current flowing between the electrodes of the lamp changes direction as a result of the alternating current (AC) supplied by the ballast to the lamp. Thus, current flows in one direction for a period of time and then in the opposite direction for another period of time. The time in which the current flows in one direction is related to the frequency. This process is controlled by the ballast and is constantly repeated during the operation of the lamp.
Without the use of alternating current and the ballast, the operation of the lamp would not be feasible. Consider that the plasma formed in the tube is analogous to an electrical short circuit. If the current only flowed in one direction the current demanded by the lamp would become enormous after only a short time. But using an alternating current only allows the current flowing through the lamp to build for a short period of time before it reverses direction and flows the other way (depending on the current's frequency). Further consider that in order for the current to reverse its direction, it must first come to a stop before it changes direction. In this way, the current demanded by the lamp and conducted through the plasma (essentially a short circuit) is prohibited from building to an unmanageable level.
Although the use of alternating current greatly facilitates the operation of gas discharge lamps, one undesirable consequence associated with such use is a phenomenon known as striations. Striations are shifting zones of light intensity appearing as dark and light bands along the length of the tube and result, in part, from the alternating nature of the current supplied to the lamp. Sometimes the striations appear as standing waves and sometimes they appear as propagating along the length of the lamp tube. Striations produce a visible strobing effect that is objectionable to many persons.
For exemplary purposes, striations can be considered in terms of the interaction between the visible light waves emitted from the lamp. Light waves are emitted according to the alternating current flowing through the lamp. If the current flowed through the lamp at a single frequency, it is more probable that light waves would interact to produce striations because there is a higher probability that the troughs and crests of the various waves, having identical frequencies, would align (thereby creating striations). However, if the current waveform flowing through the lamp contained many frequencies, i.e. the current waveform was rich in harmonics, it is less probable that noticeable striations would occur. This conclusion logically follows as waves with different frequencies have troughs and crests occurring at disparate rates and for disparate durations and these distinctions make it less likely that the troughs and crests will interfere to produce striations.
Various attempts have been proffered in the prior art to eliminate or reduce the occurrence of striations. For example, U.S. Pat. No. 5,001,386 issued to Sullivan, et al. and U.S. Pat. No. 5,864,212 issued to Sullivan disclose a dimming circuit that reduces striations by introducing an asymmetric current waveform flowing through the lamp. The asymmetric current results from a DC offset being provided along with the alternating current source.
U.S. Pat. No. 5,034,660 issued to Sairanen and U.S. Pat. No. 5,369,339 issued to Reijnaerts teach eliminating striation by inducing a direct current component within the lamp input signal. This component is introduced where the circuit design mandates that the current amplitude in one direction is necessarily higher than in the other.
U.S. Pat. No. 5,760,541 issued to Stavely, et al. describes improving longitudinal stability of intensity striations within a fluorescent lamp by causing periodic non-uniformity in the electric field between two electrodes.
U.S. Pat. No. 5,994,843 issued to Kataoka, et al. teaches preventing striation through asymmetric current flow, such asymmetry being introduced by setting the capacities of alternately charged energy accumulating capacitors to slightly different levels.
U.S. Pat. No. 6,069,453 issued to Arts, et al. discloses a ballast circuit for reducing striations by generating a lamp current comprised in part of a direct current component.
U.S. Pat. No. 6,087,785 issued to Hsieh teaches a strategy to break up elements that cause striations, in this case acoustic resonance, by modulating lamp current with a harmonized circuit to uniformly spread lamp energy into every harmonic.
U.S. Pat. No. 6,465,972 issued to Kachmarik, et al. describes a lighting system for a gas discharge lamp that eliminates striations by periodically modulating the amplitude of the lamp input signal prior to being received by the lamp.
U.S. Pat. No. 6,756,747 issued to Hsieh and U.S. Patent Application 20040085031 filed by Hsieh disclose a method for reducing striations in a fluorescent lamp by producing and alternately modulating a pair of complementary pulse trains in light of control signals that render the pulse trains asymmetrical at voltages where striation is most likely.
U.S. Pat. No. 6,836,077 issued to Nerone teaches eliminating visual striations with an asymmetric alternating lamp input current. Such a current is produced by configuring an inverting ballast circuit with complementary switches having unbalanced on times.
U.S. Pat. No. 6,963,176 issued to Onishi, et al. describes a method of suppressing striations by superimposing a pulse voltage to the voltage applied across a discharge lamp after lighting has started.
U.S. Application No. 20050168171 filed by Poehlman discloses a method for controlling striations by generating asymmetric lamp current with an unbalanced circuit component in the electronic ballast and subsequently supplying that current to the lamp.
In summary, the prior art discussed above discloses a variety of methods for reducing or eliminating striations in gas discharge lamps, most of which include either inducing or injecting a direct current component in the lamp current, modulating the amplitude of the current waveform, or creating an asymmetric frequency by modulating switch times on the inverter. The prior art discussed above also teaches eliminating acoustic resonance by spreading the harmonic energy in the lamp with a load circuit designed to generate compensating currents and modulate the lamp input current.
Although the prior art is replete with attempts to reduce or eliminate striations, none of the prior art teaches a method of operating a gas discharge lamp to eliminate striations that includes capacitive energy compensation to the lamp. What is needed, then, an effective, simple, and efficient method and system to significantly reduce and/or eliminate striations in gas discharge lamps by introducing harmonics into the current waveform by capacitive energy compensation.